The invention relates to apparatus for converting analog signals, more particularly pulse amplitude modulation (PAM) signals, into digital signals, more particularly pulse code modulation (PCM) signals, and for converting the digital signals into analog signals. The invention is particularly useful in telecommunication installations having telephone subscriber stations, each station comprising a transmitting unit supplying analog signals and a receiving unit receiving analog signals, and which throughout the duration of pulses of pulse trains, with pulses recurring in successive cycles in pulse frames, can be connected to signal receiving units receiving digital signals or, as the case may be, to signal output units supplying digital signals.
It is known to use analog to digital converters for the foregoing form of operation, as shown in commonly assigned U.S. Pat. No. 4,056,820 filed on July 27, 1976 and issued to Hofer on Nov. 1, 1977. In the operating mode of the analog-to-digital converter as described in the above patent, and particularly of the counter associated therewith, it is possible that a digital signal to be converted into an analog signal will appear within an ongoing counting cycle of the counter at a time when the digital signal cannot be accepted immediately in a buffer register of the analog-to-digital converter. Thus, it becomes necessary to use an input register at the input of the analog-to-digital converter receiving the digital signals. Due to its buffering function, the buffer register must be a static input register until the reception of an accepted digital signal. Although such a static input register, in the case of conversion of digital signals with serial bits into analog signals, can at the same time be shared for a necessary serial-parallel conversion of these bits, it is sometimes desirable to manage with a simpler serial-parallel converter, particularly with a dynamic serial-parallel converter such as, for example, with a delay line to which are applied at one end the serial bits of the digital signal and which supplies the bits of the digital signal on a number of equidistant taps corresponding to the number of bits making up a digital signal.
It is, therefore, an object of the invention to provide a means for the conversion of digital signals into analog signals in accordance with the system shown in U.S. Pat. No. 4,056,820 without the buffering of digital signals in a static input register and for the conversion of digital signals with serial bits into analog signals with a simple dynamic serial-parallel converter.